The changing of the quality of water in lakes and rivers from clear and pristine to murky, green and foul smelling is a natural process called eutrophication. Unfortunately, man's activities have speeded up this process to such an astonishing rate that the changes which would naturally take thousands of years are now noticeable in a matter of a few years and even months. These trophic changes are triggered by the introduction of large quantities of nutrients such as phosphorus and sulfur into the aquatic environments which radically alters the ratio of these elements with the natural systems that have kept them in check.
As nutrients, phosphorus and sulfur promote the growth of a large group of aquatic organisms such as algae as well as many of the higher plants. The threshold level of these nutrients which can trigger this rapid growth appears to be very low, 30 parts per billion (ppb) or less. Oligotrophic, or clear, clean lakes have nutrient values below these levels, whereas, hyper-eutrophic ponds and lakes can have values above 350 ppb. and are characterized by algae, plant growth and, in many cases, the rotten egg smell of hydrogen sulfide.
The use of algicides and aquatic herbicides to control rampant plant populations in lakes is at best an expensive band-aid which must be repeated, several times per year in many cases. Other nutrient controls, such as alum, are extremely expensive, technically demanding and have long term side effects. Unfortunately, it is not merely enough to stop the introduction of new nutrient loads into these environments since the existing nutrients are continually recycled as plants die and decay.
In addition to the changes in trophic state the impact of toxic heavy metals from a variety of sources has left many aquatic environments hazardous to all living systems.
This invention in one aspect relates to methods for controlling sulfur phosphorus and heavy metals in aqueous/sedimentary systems by aquatic nutrient binding and precipitation. Phosphorus as phosphate and sulfur as sulfide are removed by iron binding and precipitation. Heavy metals are removed by replacement reactions in aqueous carbonaceous systems which supress the heavy metal solubility and migration, thus restricting the heavy metal bioavailability. More particularly, in this aspect, the present invention involves methods for controlling excessive epiliminetic bioavailability of sulfur ions and phosphorus ions in aquatic/sedimentary environments such as lakes, ponds and waste treatment systems. Such ions are considered to be "nutrients" in aqueous environments with respect to vegetation, algae and the like. It also controls heavy metals including Hg, Pb, Cu, Ni, Zn, Cr and Mn by direct replacement and precipitation of these less active metals. While particularly focusing upon aquatic environments, the methods of this invention in this aspect are likely to be useful in essentially any aqueous/sedimentary system where control, usually reduction, of the concentration of phosphorus or sulfur ions, among other things, in all of their various oxidation states is desired.
Excessive quantities of sulfur and phosphorus compounds are becoming more prevalent in the environment. These elements are byproducts of many activities which enhance quality of life. Many biological processes also generate these ions, in free or combined form. Excessive concentrations of these elements in the free or combined states in aquatic systems tend to create problems including the aforementioned aquatic eutrophication, objectionable odors and environmental toxins.
Elemental sulfur (that is, sulfur with an oxidation state of zero, S.sup.0) can exist in both oxygenated (aerobic) and deoxygenated (anaerobic) aqueous or aquatic environments. Elemental sulfur also can be activated by oxidation (e.g., in aerobic environments) to sulfate (SO.sub.4.sup.-2) ions; and by reduction to sulfide ions (S.sup.-2). Sulfur in all oxidation states can be utilized to form organic sulfur compounds. Organic sulfur compounds are important in protein formation and many other biochemical processes. As the microbes, plants, and animals die and decay or as the waste byproducts of these organisms decay, sulfide ions (S.sup.-2) and bisulfide ions (HS.sup.-1) are released into the environment.
Anaerobically, sulfide binds to a number of metals to create sulfides in aquatic environment sediments and at the water/sediment interface. Metal sulfides have a very limited solubility in water. That is, they have very small solubility constants or solubility products in water. Thus, metals which have become bound to sulfide and bisulfide ions are removed from the aquatic environment or aqueous systems by precipitation. Iron (II) Fe.sup.+2 is the most prevalent cation, in the list of decreasing ionic activity which forms a tightly bound sulfide compound. The heavy metal sulfides have much lower solubility products than iron and so are depleated from the water column in the presence of excess ionized iron. As the aquatic system moves to a stable condition in the sediments, the solubilized sulfide ion will move to the condition of least activity. The most common cationic residence of sulfide is as iron sulfide (ferrous sulfide) which has a solubility constant of 1.times.10-17 moles per liter.
In most of the trophic states, phosphorus, as phosphate (PO.sub.4 and PO.sub.3), is the limiting element for biological productivity. Phosphate ion is precipitated by a number of metal ions but is later released from sediment by the exchange of sulfide ion with the phosphate ion and causing a reprecipitation. The sulfide is then retained on the metal and the phosphate is released to the water column as an available biological nutrient. In natural oligotrophic lakes, the sulfur, phosphorus, and iron, in the sediments, are in a balance which prevents the release of either sulfur or phosphorus to the free water as ionic nutrients. These aquatic biological systems are operating on limited nutrient budget. In this manner, they maintain clear water and thinly populated vegetation beds.
There are many aquatic systems where there is a large excess of free sulfide ion at the sediment interface and in the anoxic hypolimnetic waters. Without iron to precipitate this sulfide the subsequently released phosphate will migrate to the aerobic bioactive regions. This rate can be determined by its productivity level, its subsequent annual detritus deposition rate, and the bioavailability of the deposited nutrients. Detritus is the combined residual accumulation of microbial, plant, and animal materials. There are four recognized trophic states or bands: oligotrophic, mesotrophic, eutrophic, and hypereutrophic. The oligotrophic state is characterized by extremely low biological productivity and a very small bioavailable nutrient pool. "Mesotrophic" describes an actively recycling aquatic system but growth limited by general nutrient availability. "Eutrophic" describes a continuous biologically productive aquatic system which is utilizing a large nutrient pool to the exclusion of one or more nutrients. "Hypereutrophic" describes an out-of-control biological system in which productivity is limited by environmental conditions and has an available nutrient pool which cannot be fully utilized.
As phosphorus and sulfur are added to an aquatic environment, this desirable balance of a lake can be upset. The iron content of the incoming water does not keep pace with the increasing levels of phosphorus and sulfur because, unlike phosphorus and sulfur, iron is not a significant natural waste byproduct of modem lifestyles or common biological processes.
As the sulfur and phosphorus concentrations in the sediments become greater than the quantity of iron available to retain them, these elements are cycled or released into the upper aquatic environment. The result is increased growth of aquatic plants, epiphytic algae, and planktonic algae. Sulfur and phosphorus then become available for accelerated and unrestrained biological activity which further hastens the eutrophication process. Once the release has begun, the resulting microbial activity further enhances the release process.
Inorganic and organic sulfur compounds are readily reduced to sulfides by microbial and other environmental processes in anoxic environments. These sulfides are toxic to many biological organisms even in very small concentrations. In higher concentrations, sulfides can prevent seed germination in, for example, wild rice, and probably many other native plants. Free sulfide ions are chemically able to replace the phosphate ion on a ferrous iron molecule and thus reintroduce phosphate into an aquatic environment from the bottom sediments. The ability to control the sulfide and selectively control the soluble reactive phosphorus in an aquatic environment becomes a very useful tool for the management of the water environments mentioned above. That in one aspect, is what the subject invention is about and it makes use of ferrous iron (Fe.sup.+2) and ferric iron (Fe.sup.+3) for this purpose in combination with microbe(s) mediation.
In another aspect and in addition to the above uses, this invention provides a method for oxidizing organics to deplete the carbon pool which may include hydrocarbons and pesticides, from the sediment of aqueous systems and from the soil generally. As such, the method may be used not only to reduce the sediment depth in lakes ponds and the like by removing organics but may be used to remove unwanted hydrocarbons and the like from soil as well.
Current efforts at managing objectionable levels of nutrient concentrations in all types of aquatic environments involve the use of everything from cosmetic coverup to nutrient inactivation schemes involving various phosphorus precipitating agents. Some of these approaches are as follows:
1. Harvesting of aquatic vegetation is used as a means of controlling aquatics such as Eurasian milfoil and other large aquatic plants on recreational lakes. The harvesters are extremely expensive to buy and operate, leave a residue of aquatic vegetation which is free to populate new areas, and require handling and disposing of large volumes of vegetation which has a very low concentration of the target nutrients. A thousand pounds of vegetation contains one pound of phosphorus and a much smaller amount of sulfur.
2. Utilization of herbicides falls under the category of cosmetic treatments. This approach has a very short term positive effect and potential long term negative effects. Aside from the potential damage to non-target systems, there is the buildup of decaying algae and or vegetation as detritus which later becomes a nutrient source for subsequent growth.
3. Copper sulfate treatment for algae is yet another technique which is also largely cosmetic. This technique has the drawback of putting copper, a required micronutrient for the growth of undesirable blue-green algae, into a system where it was not readily available prior to the treatment.
4. Diatomaceous earth is used as a nonspecific flocculent for algae. It has a high cost and is restricted to binding positively charged molecules. The result is a lowered total alkalinity and hence a reduced capacity for the aquatic system to remove the unwanted sulfur and phosphorus ions.
5. Alum (aluminum sulfate) as a specific precipitant for phosphorus as phosphate and a general flocculent for algae, microbes, and organic molecules. Alum treatment of an aquatic system is quite expensive because the process requires titration of the total alkalinity of the water to a pH of 6.5 before the aluminum can effectively act on the target phosphorus. The treatment is short lived as the phosphorus is released exponentially as the pH of the water rises above the 6.5 point. An additional problem with the alum treatment is that the sulfate which, is left in the solution will later become sulfide under anerobic microbial activity and aid in the release of the phosphorus which has been flocculated. Eventually alum treatments can no longer precipitate phosphorus from the water column.
6. Ferric chloride is used as a water treatment chemical as a general flocculent. The cost per pound of phosphorus removed is comparable to that of the alum and it leaves free chloride ions in the water. Free chloride ions interact to release the phosphorus from the sediments. The more chloride, the poorer the sediment retention of phosphorus and sulfur, and the more difficult it becomes to cause the precipitation.
7. Ferric sulfate is used in much the same way that ferric chloride is used with the residual problems of the sulfate contributing a substantial quantity of sulfide ion in the sediment. Since the sulfur is used in equal quantities with the iron, there is likely to be no net positive long term effect.
8. Aeration is often used as a means of controlling the recycling of phosphorus in aquatic systems by maintaining the available iron in the system in the ferric state (Fe.sup.+3) where the solubility product is about 1000 times lower than the ferrous state iron (Fe.sup.+2). The available iron is then able to bind more phosphorus at the sediment interface. The limit here is the quantity of available iron and the maintenance of the sulfur as sulfate ion. The system is expensive to install and operate since all the effective systems are electric motor driven. Also, open water must be maintained throughout the year to sustain the gains as an artificial aerobic environment which will crash if the air flow, which drives the systematics, is not sustained thus creating a serious safety danger of someone falling through the thin ice of lakes in the northern regions.
U.S. Pat. No. 4,008,169 to Patrick John McGauley discloses a preparation of iron oxide sorbent for sulfur oxides. The McGauley patent discloses a method for preparing, at elevated temperature, an active iron oxide sorbent which is used to remove sulfur oxides from, e.g., omission gases. No mention is made of the removal of sulfur from aqueous media.
U.S. Pat. No. 4,202,864 to Jerome S. Spevack discloses a process for removing, intre alia, hydrogen sulfide from steam. The '864 patent discloses the process of contacting the impure steam with an aqueous dispersion of at least one metal compound which is capable of reacting with the hydrogen sulfide and forming a solid metal sulfide reaction product, for example an iron, zinc or copper compound. No mention of treating aqueous media is made in the '864 patent.
P. J. Dillon et al. in "Retention and resuspension of Phosphorus, Nitrogen and Iron in a Central Ontario Lake", 47 J. Fish., Aquat. Sci, 1269 (1990), disclose the monitoring of phosphorus, nitrogen and iron sedimentation in a Canadian Lake. No mention is made of removing excessive amounts of any particular species.
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The present invention is neither disclosed nor suggested, alone or in combination, by any of the prior art referred to above.